The manufacturing of semiconductors regularly employs the use of chemical solvent baths, i.e., acids, bases and organic solvents, for both cleaning the semiconductor wafer and removing unneeded portions of layers, e.g., photolithographic masks, formed during an intermediate process. Especially in acids or strong bases, certain transition metals, such as copper (Cu), iron (Fe), and zinc (Zn) may be soluble as ions in the solvents used. Unfortunately, traces of these metals in an acid etch or solvent cleaning solution can precipitate onto critical areas of the semiconductor wafer during processing, resulting in failure of the integrated circuit. As the concentration of a particular metal ion increases in the solvent, the likelihood of contamination of the wafer increases. Heretofore, the only available method of monitoring metal ion concentration in the solvent has been by sampling the solvent and analyzing the sample ex situ, while the processing of semiconductor wafers continues. The major problem with this procedure is that the wafers processed while the solvent testing is being performed are at risk of contamination and failure.
Historically, copper has not been a major problem in it semiconductor manufacturing. Aluminum (Al) has been the metal of choice in the formation of interconnectivity traces in the semiconductor back-end. It was only recently that the problems associated with using copper in semiconductor manufacturing were solved. However, with the recent advances in copper technology, the movement in the industry will certainly be toward greater use of copper because of its superior conductive properties over aluminum. Therefore, any metal ion monitoring method must address the introduction of copper to semiconductor manufacture.
Accordingly, what is needed in the art is an in situ, real time, monitoring system for transition metals in semiconductor processing chemical baths.